The technologically useful properties of a solid often depend upon the types and concentrations of the defects it contains. Defects affect the effectiveness of catalysts including photocatalysts, the performance of photo-active devices, the sensitivity of solid-state electrolyte sensors, and the efficiency of devices for converting sunlight to electrical power. Recent work in our laboratory has identified two new methods for defect engineering in silicon, based on surface-bulk coupling and optical stimulation. Similar physics should also operate in other semiconductors such as metal oxides. TiO2 is widely used as a catalyst and photocatalyst. Charged defects are known to exist in TiO2, which means that surface-bulk coupling and optically stimulated diffusion of bulk defects should be likely to occur under suitably chosen conditions. Thus, understanding the basic defect structure in TiO2 and being able to control it, provides a handle for improving processing and performance of these catalysts. In TiO2 nano structures, the proximity of the bulk to the surface can be taken advantage of to manipulate the bulk defects and hence the surface catalytic properties. A model describing the reaction and diffusion of the various point defects and vacancy clusters in TiO2 has been developed and implemented using the continuum simulator FLOOPS. The model comprises Fickian diffusion, electric drift motion, cluster formation and dissociation as well as the exchange between defects and the lattice, with the governing activation energies obtained from literature. Defect-mediated diffusion of isotopically labeled Ti (mass 46) in crystalline TiO2 measured through SIMS profiles will be used as a tracer to study the formation and diffusion of defects. The work will discuss the possibilities for demonstrating surface and optical phenomena in TiO2.